Cavitation control means for blade periphery



June so, 1936. F DY 2,045,918

CAVITATION CONTROL MEANS FOR BLADE PERIPHERY Filed Dec. 29. 1935 INVENTOR LFMooafy Patented June 30, 1936 4 PATENT OFFICE CAVITATION CONTROL MEANS FOR BLADE PERIPHERY,

Lewis Ferry Moody, Princeton, N. ',I., assignor to Baldwin-Southwark Corporation, a corporation of Delaware Application December 29, 1933, Serial No. 704,451

4 Claims. (01. 253-117) This invention relates generally to improved' means for reducing or eliminating cavitation at the periphery of unshrouded blades of rotary hydraulic machines, such as hydraulic turbines or pumps, although the invention is especially described in its application to turbines.

Cavitation has been one of the major problems with rotary hydraulic machines, and various arrangements and solutions have heretofore been proposed for controlling or eliminating cavitation at those parts of the equipment where cavitation is most likely to occur.

My present invention has particular reference to the cavitation at the periphery of unshrouded runner blades, and it is one object of my invention to provide improved means for reducingcavitation at this point. A further object is to provide a relatively simple arrangement which will not only function to reduce cavitation but.

also reduce leakage between the runner periphery and the surrounding stationary passage wall.

A more specific object of the invention is to provide improved means whereby a double pressure reduction occurs as water flows through the clearance space around the runner periphery, the peripheral portion of the blades being recessed to deliberately provide a region of eddy currents at a point Where cavitation most likely occurs.

- Specifically this is accomplished by slotting the peripheral edge of the runner blades while maintaining the normal peripheral clearance at' the face and back of the blades.

Other objects and advantages will be more apparent to those skilled in the art from. the following description of the accompanying drawing in which:

Fig. 1 is a diagrammatic vertical sectional view of one form of propeller runner embodying my invention;

Fig. 2 is an enlarged fragmentary sectional view of a blade and a surrounding passage wall showing the manner in which internal eddy currents are formed at the periphery between the face and back surfaces of the blade;

Fig. 3 is a modified arrangement of peripheral grooves whereby three pressurereductions occur;

Fig. 4 is a further modification of the peripheral grooves whereby the length of each narrow clearance passage is reduced to zero by the use of triangular grooves.

In the particular embodiment of the invention which is disclosed herein merely for the purpose of illustrating one specific form of the invention among possible others that it might take, I have diagrammatically shown a conventional type of rotary hydraulic machine specifically a turbine having a head cover I, adjustable inlet guide vanes 2, a transition space 3 inwhich thewater'is turned from a radial toward an axial direction to flow through an unshrouded runner generally; 5 indicated at 4. This runner is disposed within a suitable passage 5 whose stationary wall 6pref= erably, but not necessarily, of cylindrical form,

has a close running clearance with the periphery m,

'l of each of the several runner blades 8. The 10 runner may, of course, be of any usual form whether of the axial or diagonal flow type or-it may have adjustable or non-adjustable blades. In any case, however, the peripheraledge of the blades is provided with a groove or slot generally indicated at 9. This groove may extend for the full peripheral length of each blade as diagrammatically indicated by line 9a. or may as shown at to in Fig. l terminate slightly short of the leading and trailing edges it and II respectively. 2

This peripheral groove may be of any sufiicient depth to accomplish its purpose, but it is preferably of suflicient depth that the inner surface l3 is not within the range of the eddy currents indicated within the groove by arrows in Fig. 2. However, if the design of the runner blades is such that a groove of the foregoing depth cannot 'be conveniently used, then the groove may be made more shallow, while at the same time obtaining the general advantages of my improved o arrangement. Preferably the ratio of the depth of groove to the width of clearance shouldbe at least 2 to 1, or in another aspect, the groove should have a width at least equal to its depth.

As a consequence of my improved arrangement it is seen that a double clearance is provided, one at M and the other at l5,'with the result that the hydraulic fluid will undergo one loss of head due to sudden enlargement in passing through clearance It into groove 9 and a second loss of head 40 in passing from groove 9 through clearance l5 to the draft tube 5. At each point of enlargement velocity head is dissipated in eddies, thus reducing the efi'ective head causing the leakage flow. Hence there is a material reduction in the velocity and leakage of the fluid through the peripheral clearance and this is accompanied by a lower velocity and higher pressure within groove 9 than would occur if the peripheral edge of the blade were straight or continuouswith only a single pressure reduction. Also by my improved arrangement the surface I3 may if desired be positioned clear of the region of high velocity and intense eddy currents within groove 9, thus reedge of the blade, although as previously pointed out this surface may be moved nearer to the blade periphery and due to the pressure built up within the groove caused by the double reduction at clearances I4 and I5, cavitation will be materially controlled without excesive detrimental effects.

In the modification shown in Fig. 3, I have multiplied the effect of theFig; 2 form by providing a pair of grooves l6 and I1, thereby providing pressure reductions at clearances i8, i9 and 20. This modified arrangement is applicable where the runner blade thickness is sufficient to permit the use of two or even more grooves. Near the ends of the blade periphery where the blade is relatively thin, one of the grooves may terminate and a single gro ve continue.

In the modiflcatiom of Fig. 4, the grooves are made triangular, cutting away all or nearly all of the parallel narrow clearance passage, and controlling the leakage flow entirely by a series of enlargements. Here the peripheral surface a subject to cavitation has been eliminated, the flow being required to pass a series of relatively sharpcorners where the velocity past the corner is made to enter an enlarged chamber in which a part of its kinetic energy is dissipated eddies without harmful effects.

From the foregoing disclosure it is seen that I have provided an extremely simple and yet very effective means for reducing or eliminating cavitation at the blade periphery, this being accomplished without weakening the blade in any way and at Y the same time reducing the leakage through the clearance space. The reduction of the velocity of leakage flow through clearance It reduces the tendency toward cavitation on the under side of the blade near the peripheral edge due to the eddy produced by this leakage flow as it leaves the clearance space.

It will of course be understood that various changes in details of construction and arrange-.

ment of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

I claim:

L'A hydraulic machine having a runner and inlet and outlet passages therefor wherein water pressures above and belowatmosphere normally exist on opposite sides of the runner, said runner having a hub and a series of separate unshrouded propeller blades projecting from said hub at spaced points on a circle thereof, and each of said blades being provided with a normally peripheral groove formed as a recess between the 10 front and back blade surfaces to control cavitation at the blade peripheries.

2. The combination set forth in claim'l further characterized in that said blade peripheries are each provided with a plurality of grooves thereby to eflect mutiple contractions and enlargements of the passage for leakage flow between the runner periphery and surrounding passage wall.

3. A hydraulic propeller type turbine having an unshrouded propeller runner disposed within a passage which comprises an inlet and draft tube and has a wall immediately surrounding the runner periphery with clearance therebetwee the peripheral edge of said runner blades having a normally open groove therein providing a recessed peripheral surface disposed entirely be-' tween the face and back surfaces of the blade,

to control cavitation at the blade periphery, said recessed surface being set back from the region of high velocity leakage flow passing through said clearance.

4. A rotary hydraulic'machine having unshrouded blades, a stationary passage wall im- 

